1. Field of the Invention
The present invention relates to a PROM built-in micro computer having a semiconducting nonvolatile memory which can be written electrical on a single chip, and having a micro computer.
2. Description of the Related Arts
The PROM built-in micro computer having the semi conducting nonvolatile memory which can be written electrically and the micro computer has been widely used in electrical apparatus including home electrically apparatuses.
Up to now, a readout margin has been reserved, by providing an operating supply voltage Vdd which is higher than a specified value at a time of a data comparison after a writing operation, in order to prevent a readout error resulting from a fluctuation of a supply voltage or a temperature in a micro computer having a semiconducting nonvolatile memory (hereafter called PROM) which can be written electrically.
The method uses a fact that the readout error does not appear if the Vdd is higher than the specified voltage (for example 6 V) in order to prevent the readout error within an operating temperature (for example -20.degree. C. to 70.degree. C.) in the specified voltage of the Vdd (for example 4.5 V to 5.5 V), based on a characteristics of a PROM cell. Then, in the method, it is possible to reserve the margin, if the Vdd is higher than the specified value, in a case where the Vdd is higher than a constant level. However, if the Vdd is reduced below the constant level, it is difficult to prevent the readout error resulting from a reduction of a readout current of the PROM cell.
Recently, there has been a problem of preventing the readout error in a case where the supply voltage is low, with relation to a reduction of the operating supply voltage of the micro computer in which the PROM built-in micro computer is built in.
In order to solve such a problem, for example, there is a method (Japanese Patent Application Laying Open (KOKAI) No. 4-103096) and the like of comparing a data in the PROM with a data in another memory which is insured to be operable until a low voltage, by reducing the operating supply voltage at the time of the data comparison.
In the prior art the readout error can not be prevented for the PROM built-in micro computer which is low in a supply voltage. In order to solve the problem, if a method is tried to compare a data in the PROM with another memory which is insured to be operable until a low voltage in the PROM, however, it is difficult to realize it by using a reference PROM writer because of a difficulty of a readout data processing. Therefore, in order to realize it, it is necessary to add a large circuit inside a LSI.